This invention relates to a cryostat which contains cryogenic liquid such as liquid helium with a double radiation shield cooled by a refrigerator.
A cryostat with a double radiation shield cooled by a two stage refrigerator for minimizing the evaporation rate of the cryogenic liquid is disclosed in the Japanese Patent Disclosure (Kokai) No. 60-69540. The first stage of the refrigerator operated at a temperature of about 80 degrees K. has thermal contact with the outer radiation shield, while the second stage operated at about 20 degrees K. has contact with the inner radiation shield. Such a refrigerator requires removal from the cryostat once in a while for maintenance and/or replacement.
The refrigerator shown in the Japanese patent disclosure has two metal-to-metal surface contact portions, one at the first stage and the other at the second stage. With such a construction, it is difficult to obtain sufficient thermal contacts simultaneously at the two contact portions, and the removal and the re-installation of the refrigerator is a time-consuming job. Furthermore, the entire cryostat must be warmed to room temperature before the refrigerator is removed so that the humidity in the air can not condense and freeze on the radiation shields and on the contact portions. Consequently, the refrigerator removal and reinstallation operation consumes an extensive amount of liquid helium, as well as taking a long time.